A diesel engine or other internal combustion engine burns fuel in the engine body and produces exhaust which contains pollutants. The pollutants of exhaust gas include carbon monoxide (CO), unburned hydrocarbons (HC) and particulate matter (PM) and also nitrogen oxides (NOX). As one method which removes nitrogen oxides, it is known to place a device which reduces the NOX in the engine exhaust passage.
Devices which reduce NOX include an NOX storage reduction catalyst which temporarily stores NOX. An NOX storage reduction catalyst stores NOX when the air-fuel ratio of the exhaust gas is large, that is, when the air-fuel ratio of the exhaust gas is lean. As opposed to this, when the air-fuel ratio of the exhaust gas is small, that is, when the air-fuel ratio of the exhaust gas is rich, it releases the stored NOX. The NOX is removed by reduction by a reducing agent which is contained in the exhaust gas.
Japanese Patent Publication (A) No. 2004-84638 discloses a method of treatment of engine exhaust gas which includes a step of using a plasma generator to convert part of the exhaust gas components to an oxidant component and uses the oxidant component to make the carbon component in the exhaust gas oxidize and thereby produce carbon monoxide and a step of reducing the NOX in the exhaust gas by the reduction action of carbon monoxide on a denitridation catalyst.
Japanese Patent Publication (A) No. 2006-57478 discloses a device for regeneration of an exhaust purification member which is provided with a burner which injects combustion gas at the upstream side of the NOX storage reduction catalyst. This regeneration device makes fuel incompletely burn at the burner and injects combustion gas made to increase in the content of carbon monoxide or the content of fuel gas so as to regenerate the exhaust purification member.
Further, devices which reduce NOX which is contained in exhaust gas include an NOX catalyst which causes continuous reaction of NOX and a reducing agent.
Japanese Patent Publication (A) No. 2001-20720 discloses an exhaust purification system which is provided with a filter which is arranged in an exhaust passage of a diesel engine and a weak oxidizing strength catalyst and NOX reduction catalyst which are carried on the filter and which arranges a weak oxidizing strength catalyst at an upstream side of the NOX reduction catalyst. In the exhaust which passes through the filter, the weak oxidizing strength catalyst causes partial oxidation of the hydrocarbons to be promoted resulting in the carbon monoxide and aldehyde ratio becoming higher. Further, it is disclosed that by this exhaust passing through the NOX reduction catalyst, a high reduction efficiency of nitrogen oxides is obtained.
Japanese Patent Publication (A) No. 3-72916 discloses a method of treatment of exhaust gas which passes exhaust gas through a catalyst layer by a surface area speed of 100 to 5000 m3/m2·hr to thereby selectively produce carbon monoxide from the particulate which is contained in the exhaust gas and which uses the carbon monoxide to remove the nitrogen oxides in the exhaust gas.
Further, Japanese Patent Publication (A) No. 2008-238059 discloses a device which is comprised of a catalyst, including a carrier and a chloride of an alkali metal or alkali earth metal or other catalyst component, carried on a diesel particulate filter.